1. Field of the Invention
This invention relates to a control device for controlling a plurality of servo motors and a servo motor controlling method, and in particular to a control device for controlling and driving a plurality of servo motors to move an object to a predetermined position in a mechanical coordinate system and the servo motor controlling method used in the control device.
2. Discussion of the Related Art
Machine tools conventionally have a tool for tooling a workpiece and a table for supporting the workpiece. The machine tool is equipped with a servo motor control device for driving and controlling a plurality of servo motors to move an object such as a tool or a table in any direction within X-, Y- and Z-axes that constitute a mechanical coordinate system. When the servo motors are driven and controlled in such a manner by the servo motor control device, the object is moved to any desired position in the mechanical coordinate system.
The table of the typical machine tool is connected to a servo motor that drives the table along the X-axis and to another servo motor that drives the table along the Y-axis. Under control of the servo motor control device which controls both servo motors, the table follows composite movements in the X and Y directions.
The servo motor control device conventionally comprises a commanding section for outputting a move command signal to drive and control the servo motors, and an interpolating section for performing interpolation based on the move command signal input from the commanding section, and for outputting an interpolating signal based on the interpolation. The interpolating section is capable of performing circular arc interpolation when the commanding section outputs a circular arc move command signal.
The device further comprises distributing sections for outputting distribution pulses into the servo motors each distribution pulse being based on the interpolating signal input from the interpolating section. Applying sections, which are interposed between the interpolating sections, and the distributing sections apply acceleration and deceleration time constants to the interpolating signals output from the interpolating sections and output the so modified interpolating signals into the distributing sections.
The typical device comprises comparators which subtract feedback signals, supplied by encoders equipped with each of the respective servo motors, from the distribution pulses output from the distributing sections and output a signal. Amplifiers amplify the comparator outputs for driving and controlling each of the respective servo motors.
Conventionally, the servo motors for driving the X-and Y-axes are each assigned an acceleration and deceleration time constant. That is, under servo control, the acceleration and deceleration time constant is used to soft-start and soft-stop the object being moved. This arrangement allows the mechanisms of the machine tool to start gently (soft start) and to stop gently (soft stop) so that they will not be damaged.
However, there is one disadvantage involved in using the above-described acceleration and deceleration time constant. Assume that during a circular arc interpolation situation such as is shown FIG. 4, a circular arc interpolation command is issued to move the object from point P1 to point P2 at a velocity F on a circular arc locus C0 with a radius R0 (shown in solid line). Under previous servo control, the above-mentioned acceleration and deceleration time constant remains (as its name implies) constant, regardless of the type of interpolation (circular arc or linear). Given a command signal from the servo control device, the object under control such as the tool does not move in real time; there always exists a time lag between the issuance of a command and a tool movement. When an acceleration and deceleration time constant is applied to the above circular arc interpolation, with the radius R0 significantly small or the feed velocity appreciably high, the actual locus that the tool follows is C1 with a radius R1.
The locus "shrinks" radially, leaving behind a large error .DELTA.R (=R0-R1). The radius error .DELTA.R is expressed by the equation described below: EQU .DELTA.R.apprxeq.(.tau.r.sup.2 +.tau.P.sup.2) F.sup.2 / (2.multidot.R)
where, R stands for the circular arc radius, F for the velocity, ".tau.r" for the acceleration and deceleration time constant and ".tau.P" for the time constant of the positioning system in use.
When circular arc interpolation is carried out as described above using the acceleration and deceleration time constant ".tau.r" having a large value the above-mentioned soft-start and soft-stop of the controlled operations are accomplished. This protects the mechanisms of the machine tool. However, a large radius error in operation accuracy results.
A number of solutions have been proposed to prevent the above-noted radius error. One solution is disclosed in Japanese Laid-open Patent No. 63-146108. According to this solution, the servo control device uses a different time constant of acceleration and deceleration for each of the servo motors that drive the X- and Y-axes depending on the type of interpolation being performed (linear or circular arc). In the above setup, the radius error may be minimized, for example, by setting as small an acceleration and deceleration time constant as possible for circular arc interpolation.
However, the above setup still has impediments, including the need to soft-start and soft-stop the delicate and costly machine tool to protect the mechanisms thereof. Making the time constant smaller means making the mechanisms accelerate and decelerate more abruptly and thus stressfully. Accordingly, a limit to minimizing the time constant of acceleration and deceleration must be provided in the above-noted device.
As described, the use of small acceleration and deceleration time constants only, while permitting good circular arc interpolation, will sacrifice mechanical protection of the machine tool. This will leave the machine tool vulnerable to damage of its mechanisms.